The cause of syphilis, Treponema pallidum, has never been cultivated in vitro. The inability to grow this organism in large numbers has prevented investigations leading to the development of a useful vaccine, studies on the mechanism of disease production, and the solution of complex immunologic problems associated with this disease. We propose to attempt cultivation of this treponeme in vitro by (a) obtaining preliminary information on substrate and oxygen requirements of tissue-grown treponemes, and (b) applying this information to attempts to cultivate in vitro. The latter will require the development of micro-methods to measure growth, either in terms of increases in cell numbers or evidence of macromolecular synthesis.